A circuit breaker is an overcurrent protective device that is used for circuit protection and isolation. The circuit breaker provides electrical system protection when an overcurrent condition, such as an electrical abnormality due to a short circuit or an overload event (e.g., over heating), occurs in the system. One type of circuit breaker is a miniature circuit breaker (MCB), which is typically used for low voltage applications. An MCB can include a base and cover, and an electrical path between a line terminal and a load terminal. The electrical circuit includes a conductive stationary contact electrically connected to one of the terminals and a movable contact electrically connected to the other terminal. The movable contact is secured on a movable blade (also referred to as a contact carrier). A handle interfaces with the blade and a trip lever of the trip assembly. The handle can be operated by a user to move the blade, and thus the movable contact, between an open position and a closed position (also referred to as ON position) to open or close the electrical circuit. In the closed position, the movable contact is engaged with the stationary contact to allow current flow between the two contacts to a protected load. In the open position, the movable contact is disengaged from the stationary contact to prevent or interrupt current flow to the protected load.
The MCB has a trip assembly that, when unlatched to the tripped position, causes the blade to move to an open position in the event of an overcurrent condition. The trip assembly can include a trip lever, which is connected to the blade via a toggle spring (also known as a “tension spring”), and a trip actuator to move the trip lever to the tripped position. The trip actuator can include a bimetallic member electrically connectable to the protected circuit, a yoke and a movable armature. The trip lever is latched in an opening of the armature when the circuit breaker is in the closed position. In the event of an overcurrent condition such as due to a short circuit, the bimetallic member generates a magnetic field which, in turn, generates a magnetic flux in the armature and the yoke that are separated by a magnetic gap. The magnetic flux causes the armature to move toward the yoke, thereby unlatching the trip lever from the armature into the tripped position.
The magnetic gap of the trip actuator plays a critical role in the tripping operation (e.g., trip sensitivity and timing) for overcurrent protection, particularly against short circuits. The magnetic gap is a function of the inverse square of the distance between the two parts, namely the armature and the yoke. Thus, the magnetic gap needs to be as small as possible to have the most effect on the attraction between these two parts of the trip actuator. However, the trip actuator in some existing circuit breakers is susceptible to wider part/component tolerances (e.g., part pivots, stops, etc.), which result in inconsistencies or variances in the magnetic gap due to the number and location of parts that determine the magnetic gap. These variances can result in inconsistent tripping among circuit breakers because the magnetic gap has a non-linear effect on forces that pull the armature and yoke together.